Understanding Strategic Missile Defense

What it can and can't do.

Picture: Northrop Grumman

Writing about missile technology and nuclear strategy often involves making implicit or explicit assumptions about missile defense. Many disagreements about what missile defense can or cannot achieve, whether in conventional or nuclear scenarios, stem from differences in these underlying assumptions.

Especially strategic missile defense, which is designed to intercept strategic nuclear warheads, is poorly understood outside expert circles. This is not a criticism of the general public; strategic missile defense is a very complex issue that encompasses not only technical considerations but also a wide range of political factors. This post aims to provide a short and hopefully accessible introduction to the topic.

Technical challenges

Strategic missile defense systems are designed to intercept incoming strategic nuclear warheads, typically deployed from intercontinental-range ballistic missiles (ICBMs). These warheads descend from very high exo-atmospheric altitudes, sometimes several thousand kilometers above the Earth’s surface.

To counter such threats, strategic missile defense systems require at least two-stage, but ideally three-stage, interceptor rockets capable of engaging projectiles at very high altitudes and velocities. These systems must also be supported by a sophisticated intelligence, surveillance, and reconnaissance (ISR) network—both ground- and space-based—to detect incoming threats, discriminate real from false targets, track their trajectories, and provide midcourse guidance to the interceptor until its terminal guidance system takes over in the final phase of flight.

The technical demands for strategic missile defense are significantly higher than those for non-strategic missile defense systems, for example, those optimized for short- or medium-range ballistic missile threats.

These requirements also make strategic missile defense interceptors extremely expensive. For instance, the United States’ next-generation strategic missile defense interceptor, developed by Northrop Grumman, is expected to cost at least $70 million, but more likely $90–100 million per unit. By comparison, PAC-3 MSE interceptors currently cost around $3.7 million each, while an Arrow 3 interceptor is estimated to cost between $4–5 million apiece.

Political factors

Beyond the technical challenges, there are several political factors to consider. Most importantly, in the context of nuclear strategy, missile defense is often viewed as an offensive rather than a defensive capability.

This perception stems from the fact that large-scale strategic missile defense can theoretically support counterforce or damage-limitation strategies, thereby undermining mutually assured destruction (MAD), which has been the cornerstone of strategic stability during and after the Cold War.

For example, without strategic missile defense, the United States would need to destroy 100 percent of China’s and Russia’s strategic nuclear warheads in a counterforce strike to fully deny the possibility of a retaliatory nuclear response. However, if the United States could theoretically intercept 10 percent of incoming nuclear warheads, only 90 percent would need to be destroyed in the initial counterforce strike.

While this may seem like a minor difference, it can significantly enhance the feasibility of successful counterforce or damage limitation strikes in practice. Throughout and after the Cold War, these dynamics have been seen as destabilizing, as they could incentivize an adversary to strike first during a crisis out of fear of losing its retaliatory capability. This destabilizing potential has historically led to divestments from large-scale strategic missile defense systems.

Another political drawback of strategic missile defense is the relative ease with which adversaries can counter it. It is generally assumed to be easier and more cost-effective to build additional offensive nuclear capabilities than to maintain pace with advanced defensive systems.

Personally, I would somewhat qualify this assumption by stating that context matters. At present, there is an argument to be made that the United States could potentially outpace Russia in an offense-defense nuclear arms race, albeit at significant financial and opportunity costs. However, China’s rapid nuclear expansion provides a compelling example of how a nuclear weapon state, in peacetime and with sufficient capacity, can likely always scale up offensive nuclear capabilities faster than the opposing side can develop and deploy defensive systems.

What strategic missile defense is designed to do

Due to the immense costs and political considerations associated with strategic missile defense, even states capable of deploying a mature and sophisticated system—arguably only the United States, although China appears to be catching up—keep their deployments limited.

The United States currently deploys 44 Ground-Based Interceptors (GBIs) as part of the Ground-Based Missile Defense System (GMD), optimized for engaging ICBM warhead-type projectiles. It is immediately clear that this number is insufficient to counter Russia’s arsenal of 1,500–1,600 ICBM warheads or even China’s approximately 400 ICBM warheads.

Missile defense doctrine also typically requires assigning more than one interceptor per warhead, likely at least three for purposes of strategic missile defense. Assuming each interceptor has an individual probability of intercept of 70 percent, this increases the overall probability of successfully intercepting a single target to 97.3 percent.

However, this approach also reduces the total number of targets that can be engaged. For instance, with the United States’ 44 GBIs, only up to 14 incoming warheads can feasibly be intercepted. To put this into perspective, that corresponds to the payload of fewer than four Russian RS-24 YaRS ICBMs or fewer than two Chinese DF-41 ICBMs.

This same math applies to other missile defense systems sometimes claimed to have counter-ICBM capabilities, such as the Aegis Ashore sites deployed in Europe or Germany’s Arrow 3 missile defense system, which is expected to reach initial operational capability by the end of 2025. Even if we assume an effective counter-ICBM role for these systems, their limited interceptor stockpiles—48 for Aegis Ashore and 72 for Arrow 3 at full operational capability—mean they could intercept only 16 to 24 ICBM warheads before requiring a reload.

So, what is strategic missile defense for? Essentially, its current purpose is to defend against very limited or accidental nuclear launches. It is by no means designed or optimized to counter large-scale nuclear strikes. This limitation is deliberate: anything beyond this minimum capability would likely be seen as both cost-ineffective and politically destabilizing—and arguably, with good reason.